Polymers that provide high temperature resistance, such as polyimides, are well known in microelectronic applications. Some of the uses include protective layers, insulating layers, interlevel dielectrics, and antireflective layers. In some cases, the polyimide may be coated from a solution of polyimide, but are generally coated as a polyimide precursor, such as a polyamic acid or polyamic ester, and subsequently converted to polyimides by known techniques, such as exposure to high temperatures. Frequently in the early steps of patterning applications, the properties of the polyimide precursors are more useful than the fully imidized polyimide. In the later stages, the properties of the fully imidized polyimide usually are more desired.
Preparation of polyimide patterns may use a photosensitive polyimide precursor or employ a bilayer approach where the polyimide precursor is not photosensitive. A method of patterning non-photosensitive polyimide films was described in U.S. Pat. No. 3,700,497, incorporated by reference. According to this method, a film of polyamic acid was formed on the surface of a substrate. Then, a layer of a diazonaphthoquinone/novolak type photoresist was formed on the top of the film of polyamic acid, followed by patternwise exposure using ultraviolet light. The exposed areas of photoresist and the layer of polyamic acid underneath it were etched away in a dilute organic or inorganic base developer. The remaining photoresist was removed using acetone and the film of polyamic acid was cured at temperatures higher than 250° C., creating a patterned polyimide layer to remain as part of the device structure.
The pattern profile obtained from this type of process depends on the ability to control the dissolution rate of the polyimide precursor, the thickness of the polyimide precursor, and strength and type of developer employed for the photoresist and for the polyimide precursor layers. Significant undercut of the photoresist layer can occur with high dissolution rates of the polyamic acid in the developer. This property has been utilized in preparing lift off structures from thin, sacrificial, polyimide precursor layers, which may be partially imidized, as disclosed in U.S. Pat. No. 5,360,697 and U.S. Pat. No. 5,017,459.
The dissolution rate of the polyimide precursor is a function of its chemical structure and the degree of imidization. The chemical structure of the polyimide precursor, which will subsequently form the polyimide with the properties desired for the application, may have a low dissolution rate. The polyimide precursor may be partially imidized to control dissolution rate as in U.S. Pat. No. 4,113,550. These factors may lead to development being a two-step process. First the photoresist layer is developed with one developer and then the polyimide precursor layer is developed with another developer, frequently of a different type. Two-step development processes of polyimide precursor bilayer systems are described, for example, in U.S. Pat. Nos. 4,039,371, 4,411,735, 5,374,503, 4,113,550 and 5,470,693. Typical developers employed for the polyimide precursor layer are based on amines, tetramethyl ammonium hydroxide (TMAH) in alcohol, or a mixture of tetramethyl ammonium hydroxide, water, and N-methyl pyrolidone. U.S. Pat. No. 4,039,370 discloses a developer based on tetramethyl ammonium hydroxide, water, and either acetic, tartaric or oxalic acid. U.S. Pat. No. 6,221,567 discloses a development process with multiple developer/rinse steps.
Solvents employed for polyimide precursors are typically polar solvents. Dimethyl acetamide and N-methyl pyrolidone (NMP) are the most commonly used solvents for polyimide precursors for bilayer applications. However, it has been established that nitrogen-containing solvents such as NMP have detrimental effects on the performance of chemically amplified 248 and 193 nm photoresists (U.S. Pat. No. 6,277,546; see also “Influence of Polymer Properties On Airborne Chemical Contamination of Chemically Amplified Resists”, W. D, Hinsberg, S. A. MacDonald, N. J. Clecak, C. D. Snyder, and H. Ito, SPIE vol. 1925, pp. 43–52, 1993). As a result, the use of NMP-containing compositions is prohibited in many semiconductor fabrication facilities where such chemically amplified photoresists are used. NMP is difficult to remove from the polyimide precursor film because of its high boiling point and the relatively low temperature softbakes employed with the polyimide precursor film to prevent imidization. U.S. Pat. No. 5,667,922 disclosed that a process employing a water rinse step after formation of a polyamic acid film resulted in better-defined polyimide patterns because of NMP removal by the water rinse.
The use of alternate, but weaker, polar solvents such as gamma-butyrolactone is more acceptable to the industry. However, many polyimide precursor compositions with desirable cured properties are not soluble in gamma-butyrolactone.
Non-photosensitive polyimide precursor compositions are generally employed on substrates treated with adhesion promoting agents. It is desirable to reduce the cost of semiconductor devices by reducing the number of manufacturing steps. It is therefore desirable to use adhesion promoters within the non-photosensitive polyimide precursor composition to eliminate a manufacturing step. Addition of certain silane adhesion promoters to non-photosensitive polyimide precursor compositions based on polyamic acids have been found to degrade the stability of the photosensitive polyimide precursor composition.